Auto weight painting error
I just had a look at your model. It fails to parent the base mesh properly with automatic weights and this usually happens for a number of reasons.

It's because there are errors with your base mesh. Looking around the neck you'll find quite a few holes in the mesh. These need to be filled in being careful not add interior edges or faces. 

Around the hand I found a few more holes. 

I assume, what you did was to sculpt the monkey and then decimate it.

Decimation basically decimates your mesh (destroys it). So it's no good really for animation.

Honestly the best way to approach this is to prepare your model for rigging by retopologising it correctly. So you want to avoid triangles but also make sure there are no duplicate vertices, holes in the mesh and that there are no extra parts of geometry overlapping each other. Retopology really helps us have a lot of control over our mesh to ensure that we don't have errors.

A clean quaded topology also allows for nice animation, so focus on getting nice edge loops around where the limbs fold and that the vertices are spaced out nicely and this will prevent any ugly artifacts from appearing during animation.

If you'd prefer to skip the retopology of your model (which I don't really recommend although it'll save you time), really check your mesh carefully for holes and inside faces and overlapping geometry. This actually might take more time than just retopologising it. The mesh looks quite difficult to manage as a modellor (also considering that the mesh consists of mainly triangles and many ngons).

However, to retopogise it, start with a plane and turn on snapping to face and apply a shrinkwrap modifier to the plane. Then start extruding one edge along your model. 

That's the basic explanation but I suggest checking out youtube for some good tutorials on retopology.

NOTE: After applying the Remesh modifier, I was able to Parent the mesh to the Armature with automatic weights. So it's definitely that there are some issues with the mesh.

I hope this helps you and good luck with it!